1. Field of the Invention
This invention relates generally to digital output circuits and, more particularly, to a three state output circuit.
2. Prior Art Statement
Digital output circuits typically respond to an input signal by coupling an output line to a high reference voltage via an active pull up device when the input signal is in a first logic state, or by coupling the output line to a low reference voltage via an active pull down device when the input signal is in a second logic state. In addition, three state output circuits respond to a three state control signal by disabling both the active pull up and pull down devices when the control signal is active, to present a high impedance on the output line. However, such circuits require additional components to generate the three state control signal. In addition, additional passive devices may be required between the output line and a selected one of the reference voltages to assure that the voltage on the output line will be passively pulled to the selected reference voltage whenever an active device is not actively pulling it to the other reference voltage.